1. Field of the Invention
The present invention relates to a sheet folding apparatus and an image forming system.
2. Description of Related Art
There has been developed a sheet processing apparatus provided with a sheet folding apparatus which folds sheets in three after the sheets on which printing is done are output from an image forming apparatus according to a job executed by the image forming apparatus such as a printer. Sheets are to be folded in three in a case where documents (sheets) which are larger than envelopes need to be folded to be enclosed in the envelopes to mail them out, for example.
Conventionally, as described in JP 2013-116789, a sheet folding apparatus provided with a pair of rollers which rotate and which are in contact with each other and a knife-shaped pushing unit which reciprocates linearly moving forward in its tip direction and return has been used. A nip formed of the pair of rollers is arranged in the tip direction of the pushing member, and the sheet conveyance mechanism is configured so as to convey the sheet between the tip of the pushing member and the nip. Then, the pushing member is moved toward the sheet which is conveyed in between the tip of the pushing member and the nip so that the pushing member abuts the sheet. Thereafter, by pushing the sheet in the nip which is formed of the pair of rollers, the sheet is pulled in to the pair of rollers and folded. In order to fold the sheet at a predetermined position, the pushing operation timing of the pushing member is controlled so as to be coordinated with the conveyance position of the sheet.
Folding needs to be performed at two positions in a sheet in order to fold the sheet in three, and the folding-in-three process is carried out by sequentially performing the first folding at the first position and the second folding at the second position. Therefore, the first nip for performing the first folding, the first pushing member which pushes a sheet in the first nip, the second nip which performs the second folding and the second pushing member which pushes the sheet in the second nip are provided.
With respect to the sheet folding apparatus disclosed in JP 2013-116789, total of three rollers which are the first roller which is one of the rollers forming the first nip, the second roller which forms the first nip with the first roller and the third roller which forms the second nip with the second roller are provided. Here, with any one of the first roller, the second roller and the third roller rotating by the drive unit, the other rollers rotate following the driven roller.
Further, with respect to the sheet folding apparatus disclose in JP 2013-116789, a sheet bundle is stopped and held at a stacker just before the first folding operation, the first folding position is controlled by controlling the position of the stacker in the sheet conveying direction, the first folding operation is performed by making the first pushing member move forward to the first nip where two rollers are rotating, and the second folding member is made to move forward to the second nip after a predetermined time elapsed from the time when the first pushing member started its forward movement. The second folding position is controlled by the predetermined time.
In the above described conventional sheet folding apparatus which performs the folding-in-three process, a sheet is pushed in to the second nip by the second pushing member while the sheet is being conveyed during the time from when the first folding operation ends until the second folding operation in a state where the rollers are rotating.
However, in such operation of the second folding where the sheet is pushed by the pushing member while the sheet being conveyed, since the tip of the pushing member moves closer to the sheet and comes in contact with the sheet surface which is moving in the sheet conveyance direction that is approximately orthogonal to the forward moving direction of the pushing member, the pushing member slips on the sheet and it is difficult to assure the accurate folding position even if the moving speed of the pushing member is made faster than the sheet conveyance speed.